High temperature cobalt and nickel-based superalloys are widely used to form certain components of gas turbine engines, including combustors and turbine vanes and blades. While high-temperature superalloy components are often formed by casting, circumstances exist where superalloy components are preferably or are required to be fabricated by welding. For example, components having complex configurations, such as turbine midframes and shroud support rings, can be more readily fabricated by welding separate castings together. Therefore, it is often more practical and cost effective to fabricate complex components by welding rather than casting the component as a unitary part.
In addition to the above, welding is widely used as a method for restoring blade tips, and for repairing cracks and other surface discontinuities in superalloy components caused by thermal cycling or foreign object impact. Because the cost of components formed from high-temperature cobalt and nickel-based superalloys is relatively high, restoring/repairing these components is typically more desirable than replacing them when they become worn or damaged.
Superalloy components have been preheated prior to welding and then continuously heated during welding to improve welding yields. For this purpose, preheat temperatures in excess of about 1500.degree. F. (about 815.degree. C.) and often above 1700.degree. F. (925.degree. C.) may be used. Heating and welding of superalloy components in this manner are often performed in an enclosure containing a controlled atmosphere (e.g., an inert gas) using such welding techniques as tungsten inert gas (TIG) and laser welding processes. Preheating is typically performed by induction or with the use of lamps, particularly quartz halogen lamps. Lamps heat their targeted superalloy components by thermal radiation, and are therefore often equipped with a polished aluminum reflector that reflects thermal radiation into the enclosure toward the component to be welded. When used as the heat source, a lamp is at times operated with a higher input voltage than what the lamp is rated for, e.g., a lamp rated for 90 to 100 volts might be operated at 105 to 110 volts. The result is a dramatic decrease in lamp life and less than optimal component temperature control.
Accordingly, it would be desirable if improved heating efficiency could be achieved for a welding apparatus used to perform welding operations on superalloy articles. In particular, it would be desirable if superalloy articles could be heated more rapidly and efficiently with such an apparatus, while maintaining temperature control and increasing the life of the device used to heat the articles.